Conventionally, there has been a laser processing machine as one of processing devices for cutting a workpiece having a form like a flat plate. Types of the laser processing machine are divided to one based on the fixed light axis system in which a length of a light path from a laser oscillator to a lens for converging a laser beam inside a processing head is kept constant, and the other one based on the optical scanning system in which a length of a light path from a laser oscillator to a lens for converging a laser beam inside the processing head varies during processing.
In a laser processing machine based on the optical scanning system, as a length of a light path from the laser oscillator to a lens for converging a laser beam is not kept constant, a beam diameter of a laser beam introducing into the lens for converging light changes, and for this reason a beam diameter at a converged position is not kept constant, which may badly affect a cut surface of a workpiece.
So there has been invented a laser processing machine based on the optical scanning system in which, to keep a beam diameter of an incident laser beam into a converging lens constant during processing, a laser beam oscillated from a laser oscillator is converted to a collimated beam which proceeds without diverging or converging the laser beam against the light axis and is introduced into a converging lens. Japanese Patent Laid-Open Publication No. HEI 1-166894 discloses a laser processing machine having a collimation apparatus which converts a laser beam to a collimated beam.
FIG. 14 shows configuration of the laser processing machine disclosed in Japanese Patent Laid-Open Publication No. HEI 1-166894.
The laser processing machine based on the conventional technology comprises a laser oscillator 1 forming an optical resonator by holding an excitation medium 2 for a laser with a full-reflection mirror 3 as well as a partial reflection mirror 4 and outputting a laser beam; a laser beam collimation apparatus 6B for converting a laser beam 5a (indicated by an alternate dot and dash line in the figure) outputted from the laser oscillator 1 to a collimated beam 5b (indicated by an alternate dot and dash line in the figure); a flat-surface reflection mirror 7 for reflecting the collimated beam 5b converted by the collimation apparatus 6B to change a direction of the light path by 90 degrees; a processing head 8 into which the collimated beam 5b reflected by the flat-surface reflection mirror 7 is introduced; a collimation control unit 15 for controlling a curvature of a collimation mirror 11 in the laser beam collimation apparatus 6B; a beam diameter detecting mechanism 13 for detecting a beam diameter of the collimated beam 5b introduced into a converging lens 9 inside the processing head 8; and an NC (Numerical Control) device 14, a driving mechanism (not shown) which is driven and controlled by the NC device 14.
Provided inside the collimation apparatus 6B are a collimation mirror 11 which can change a curvature R thereof and a flat-surface mirror 12 for introducing the laser beam 5a into the collimation mirror 11 at a small angle for suppressing aberration.
A curvature of the collimation mirror 11 is controlled according to a control signal S16 which is generated by and outputted from the collimation control unit 15.
When laser oscillation occurs inside an optical oscillator comprising the full reflection mirror 3 and partial reflection mirror 4, a portion thereof is oscillated from the partial reflection mirror 4 to outside of the laser oscillator 1 as a laser beam 5a. The laser beam 5a oscillated to outside of this laser oscillator 1 passes through the collimation apparatus 6B, is reflected by the flat-surface reflection mirror 7 to change its direction by 90 degrees, is introduced into the converging lens 9 in the processing head 8, is converged by the converging lens 9, and is irradiated onto the workpiece 10. The laser beam converged and irradiated onto the workpiece 10 is freely moved on the workpiece 10 by a driving mechanism (not shown) controlled by the NC device 14 to process the workpiece 10 into a desired form.
A beam diameter of the collimated beam 5b introduced into the converging lens 9 is detected by the beam diameter detecting mechanism 13. And in the NC device 14, determination is made as to whether the detected beam diameter is a desired one or not, and if it is determined that the detected beam diameter is not a desired one, a control signal S16 is sent from the collimation control unit 15 to the collimation apparatus 6B. With this feature, a curvature R of the collimation mirror 11 is changed so that a beam diameter of the collimated beam 5b introduced into the converging lens 9 will be a desired one.
FIG. 15 shows the laser beam collimation apparatus 6B based on the conventional technology. This collimation apparatus 6B comprises a disk-shaped collimation mirror 11, an actuator 17 comprising a piezoelectric element for pressurizing a central section of a rear surface of the collimation mirror 11 from the rear side, and a mirror holder 18A for accommodating the actuator 17 and collimation mirror 11 and also for fixing a periphery of the collimation mirror 11 as fixing means.
When a drive control signal S16 is sent from the collimation control unit 15 (Refer to FIG. 14) through a signal line 16 to the actuator 17, the actuator 17 presses a rear surface of the collimation mirror 11 with a force corresponding to the drive control signal S16. Because of this operation, the collimation mirror with the periphery fixed to the mirror holder 18A deforms and the curvature changes.
Herein, as shown in FIG. 15, a deflection rate W at a given radius r when a periphery of the disk-shaped collimation mirror 11 is fixed and the central portion is pressed with a load P is expressed by the following expression (1) assuming that a indicates a significant radius of the mirror and D indicates a bending component of the plate. ##EQU1##
As clearly understood from this expression (1), the deflection rate W at a radius r is not a solution of a two-dimensional function.
It should be noted that, in Japanese Patent Laid-Open Publication No. HEI 1-166894, a curvature of the collimation mirror 11 is changed by changing a pressure of gas (or liquid) applied to a rear surface of the collimation mirror 11.
However, with the laser beam collimation apparatus based on the conventional technology as described above, it is possible to change a curvature of the collimation mirror 11, as a periphery of the collimation mirror 11 is fixed to the mirror holder 18A, a large force is required to change the curvature, or a reflecting surface of the collimation mirror 11 does not become an ideal parabolic surface (Refer to the expression (1) above), and a converging capability of a laser beam becomes lower, which makes it impossible to always process a workpiece 10 under stable conditions.
When a piezoelectric element is used for the actuator 17, as only a force in a pressing direction can be obtained with the piezoelectric element, it is possible only to simply press a rear surface of the collimation mirror 11 and only to change a reflecting surface of the collimation mirror 11 into a convex mirror, in other words it is impossible to change a reflecting surface of the collimation mirror 11 into a concave mirror.
Further in the laser processing machine based on the conventional technology as described above in which change in a beam diameter due to thermal distortion of the collimation mirror 11 caused by incidence of a collimated beam (laser beam) is controlled by detecting a beam diameter of the collimated beam 5b introduced into the converging lens 9 inside the processing head 8 by the beam diameter detecting mechanism 13, the beam diameter detecting mechanism 13 cannot follow a speed of change in a beam diameter due to thermal distortion of the collimation mirror 11, and further it is impossible to always control a beam diameter of the collimated beam 5b introduced into the converging lens 9 at a constant level with high precision. In addition, it is required to provide in the processing head 8 a space for mounting the beam diameter detecting mechanism 13 thereto, and it is extremely difficult to mount the beam diameter detecting mechanism 13.